Quinacridone pigments are well known in the art and can be prepared by a number of processes some of which involve oxidizing an unsubstituted or substituted 6,13-dihydroquinacridone to the correspondingly substituted quinacfidone.
For example, U.S. Pat. No. 2,821,529 describes a process wherein various 6,13-dihydroquinacridones are oxidized to the corresponding quinacridone by heating a mixture containing the dihydroquinacridone and a mild oxidizing agent in an alkaline reaction medium. The medium is a mixture containing a major portion of an organic solvent, generally an alcohol, and a minor amount of water. The amount of water present in the reaction medium is small relative to the amount of the organic solvent.
The literature also describes processes for oxidizing a dihydroquinacridone to the corresponding quinacfidone by utilizing molecular oxygen and a quinone compound as the oxidizing agent. Such a reaction is often referred to as an "air oxidation" because air is a preferred source of the molecular oxygen. In general, such oxidation processes are disclosed as taking place in an alkaline medium, usually an organic solvent containing a minor amount of water, in the presence of a quinone compound and molecular oxygen. The molecular oxygen is introduced to the reaction medium by bubbling an oxygen containing gas through the reaction medium or by blowing the oxygen containing gas above the surface thereof. Although the literature describes the quinone compound both as a catalyst and as an oxidizing agent, U.S. Pat. No 3,024,239 discloses that the quinone is an oxidizing agent which is reduced to the corresponding leuco compound during the oxidation of the dihydroquinacridone. The molecular oxygen regenerates the quinone so that less than the stoichiometric amount of the quinone is required for the reaction to proceed to completion.
U.S. Pat. No. 3,475,436 discloses an air oxidation process wherein the reaction medium contains a major portion of tetramethylene sulfone and a relatively small amount of water. Similar processes which utilize an alkaline medium containing a major portion of other organic solvents, such as dimethyl sulfoxide, dimethylacetamide, alkanediols, C.sub.1 -C.sub.3 alcohols caprolactam and N-alkyl-2-pyrrolidone, usually in the presence of a relatively small amount of water, are also known in the art.
It is also known to perform the air oxidation of dihydroquinacridones in an aqueous reaction medium. However, the known aqueous processes are carried out in the presence of a divalent metal ion or a quaternary ammonium salt. For example, U.S. Pat. No. 3,738,988 discloses a process wherein an aqueous medium is utilized. However, U.S. Pat. No. 3,738,988 discloses that the oxidation step should be carried out in the presence of divalent iron, cobalt or nickel ions in order to increase the effectiveness of the oxidation. U.S. Pat. No. 5,093,497 requires the presence of a quaternary ammonium salt in order to overcome disadvantages in known air oxidation processes in both an aqueous reaction medium or an organic reaction medium. The present invention is based on the discovery that excellent conversion of the 6,13-dihydroquinacridone to the corresponding quinacridone is achieved in an aqueous medium via an air oxidation process if the aqueous reaction medium also contains a relatively minor amount of a nonionic, polar organic material which forms a liquid, organic second-phase in the reaction mixture. The present process provides high yields of quinacridones having superior purity compared to the product of known aqueous air oxidation processes. An additional advantage of the disclosed process is that it is not necessary to perform the oxidation in the presence of a surfactant in order to obtain a reasonable yield of high purity product. In addition, the present process eliminates the need for uneconomical quaternary ammonium compounds and metal salts, along with any potential disposal problems associated with the presence of such additives .